feeble-mindedness is inherited in a simple Mendelian fashion, although it is widely accepted as such. We can not help feeling that in most cases heredity in man is being made to appear much simpler than it really is; and that particularly in mental characters, analysis of traits has by no means reached the bottom.
If we were asked to make out a list of characters, as to the Mendelian inheritance of which there could be little doubt, we would hardly be able to go farther than the following:
The sex-linked characters (one kind of color-blindness, hemophilia, one kind of night-blindness, atrophy of the optic nerve, and a few other rare abnormalities).
Albinism. This appears to be a recessive, but probably involves multiple allelomorphs in man, as in other animals.
Brachydactyly, apparently a dominant. This is so much cited in text-books on Mendelism that the student might think it is a common character. As a fact, it is extremely rare, being found in only a few families. The similar trait of orthodactyly or symphalangism, which likewise appears to be a good Mendelian dominant, seems to exist in only one family. Traits like these, which are easily defined and occur very rarely, make up a large part of the cases of probably Mendelian heredity. They are little more than curiosities, their rarity and abnormal nature depriving them of evolutionary significance other than to demonstrate that Mendelian heredity does operate in man.
White blaze in the hair or, as it might better be called to show its resemblance to the trait found in other mammals, piebaldism. A rather rare dominant.1
Huntington's Chorea, which usually appears to be a good dominant, although the last investigators (Muncey and Davenport) found some unconformable cases.